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57 lines
2.3 KiB
ReStructuredText
57 lines
2.3 KiB
ReStructuredText
==================================
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Stack Safety Analysis
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==================================
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Introduction
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============
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The Stack Safety Analysis determines if stack allocated variables can be
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considered 'safe' from memory access bugs.
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The primary purpose of the analysis is to be used by sanitizers to avoid
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unnecessary instrumentation of 'safe' variables. SafeStack is going to be the
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first user.
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'safe' variables can be defined as variables that can not be used out-of-scope
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(e.g. use-after-return) or accessed out of bounds. In the future it can be
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extended to track other variable properties. E.g. we plan to extend
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implementation with a check to make sure that variable is always initialized
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before every read to optimize use-of-uninitialized-memory checks.
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How it works
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============
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The analysis is implemented in two stages:
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The intra-procedural, or 'local', stage performs a depth-first search inside
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functions to collect all uses of each alloca, including loads/stores and uses as
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arguments functions. After this stage we know which parts of the alloca are used
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by functions itself but we don't know what happens after it is passed as
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an argument to another function.
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The inter-procedural, or 'global', stage, resolves what happens to allocas after
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they are passed as function arguments. This stage performs a depth-first search
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on function calls inside a single module and propagates allocas usage through
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functions calls.
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When used with ThinLTO, the global stage performs a whole program analysis over
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the Module Summary Index.
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Testing
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=======
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The analysis is covered with lit tests.
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We expect that users can tolerate false classification of variables as
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'unsafe' when in-fact it's 'safe'. This may lead to inefficient code. However, we
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can't accept false 'safe' classification which may cause sanitizers to miss actual
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bugs in instrumented code. To avoid that we want additional validation tool.
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AddressSanitizer may help with this validation. We can instrument all variables
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as usual but additionally store stack-safe information in the
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``ASanStackVariableDescription``. Then if AddressSanitizer detects a bug on
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a 'safe' variable we can produce an additional report to let the user know that
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probably Stack Safety Analysis failed and we should check for a bug in the
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compiler.
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